The present invention relates to a method of reducing the halogen content in a particulate polyolefin formed by catalytic polymerization of an olefin in the presence of a Ziegler-Natta catalyst by subjecting the particulate polyolefin to a purifying treatment involving passage of gas therethrough.
In the polymerization of olefins using Ziegler-Natta catalysts, the particulate polyolefins usually obtained have a certain content of catalyst components depending upon the catalyst combination used and the polymerization conditions employed. Thus, in virtually all polyolefins of this kind there is a relatively high content of halogen, particularly chlorine. This means that the polyolefins must be treated to reduce the halogen content before they are further processed, as otherwise they would produce relatively severe corrosion in the machines used for converting the polyolefins. The presence of halogens also has a detrimental effect on the properties of the products made with polyolefins.
There are several well-known methods for reducing the catalyst and halogen contents of these polyolefins. On such method comprises dissolving the polyolefins, hydrolyzing and precipitating the catalyst components, filtering this system, and reprecipitating the polyolefins from the filtering solution. Another method consists of treating the particulate polyolefins with specific wash liquids. The third method involves the treatment of the particulate polyolefins with aqueous/alkaline solutions or sulfoxide in extruders. One highly effective extruder method is disclosed in U.S. Pat. No. 3,925,341 to Mueller-Tamm, Schick, Rau, and Hennenberger. This method involves treating the polyolefin with an alkylene oxide and water in an extruder. The halogen content is converted to alkylene halohydrin which is sufficiently volatile that it can be easily separated from the polyolefin.
Still another method for reducing the halogen content is the gas phase fluidized bed dechlorination method such as described in British Pat. No. 1,420,837 to Badische Anilin and Soda-Fabrik Aktiengesellschaft. According to that patent, particulate polyolefins are subjected to a purifying treatment involving the passage of gas therethrough, wherein the purifying treatment is carried out in a gas phase fluidized bed and the purifying gas used comprises a mixture of nitrogen, steam, and an alkylene oxide in a particular ratio. It is said therein that the method can reduce the halogen content to about 20 parts per million and it has been found that further modifications of the method can reduce the halogen content even further.
Significant amounts of alkylene oxide are consumed in the above method. Therefore, it definitely would be advantageous from a cost standpoint to recover as much of the alkylene oxide as is possible. It is well known that alkylene halohydrins can be converted to alkylene oxides by treating the halohydrins with caustic solutions. Indeed, there are a number of old industrial processes for manufacturing ethylene oxide from ethylene chlorohydrin. Those processes all utilize relatively highly concentrated solutions of ethylene chlorohydrin to produce ethylene oxide.
U.S. Pat. No. 1,446,872 to Brooks discloses a method for making ethylene oxide from ethylene chlorohydrin by reacting the chlorohydrin with a caustic alkali in the presence of as little water as possible. The patentee states that when the reaction is carried out in the presence of considerable water, very poor yields of oxide result. The patentee states that one part by weight of a solution containing 80% chlorohydrin and 20% water and 1 part by weight of solid caustic soda reacted together will produce the maximum theoretically possible yield of ethylene oxide. U.S. Pat. No. 3,886,187 to Bartholome, Koehler, Stoeckelmann, and May discloses a process for the continuous manufacture of propylene oxide by turbulent jet mixing of a propylene chlorohydrin solution with aqueous alkali, mixing with steam to produce a two-phase mixture and a special working-up process following turbulent passage through a reaction zone with a short residence time. It is said that propylene oxide is obtained in high yield and high space-time yield. The patentees state that the alkaline components are generally used in concentrations from 2 to 15 moles per liter of solution.
In the fluidized bed dehalogenation process, the concentration of alkylene halohydrin in the gas stream which is removed by volatization from the fluidized bed is very small. It has been found that when dilute solutions of alkylene halohydrin are treated with relatively highly concentrated caustic solutions such as those disclosed in U.S. Pat. Nos. 1,446,872 and 3,886,187 above, the reaction conditions favor the production of an undesirable amount of alkylene glycol rather than alkylene oxide. Aside from the fact that the production of oxide is preferred so that it can be recycled into the fluidized bed, the presence of glycol causes undesirable foaming which adversely affects the operation of the caustic scrubber where the halohydrin is contacted with the caustic solution. It has been unexpectedly found that by treating the dilute halohydrin solutions with dilute caustic solutions, the production of alkylene oxide can be maximized and the production of glycol and the foaming problems concurrent therewith can be minimized.
Therefore, it is an object of this invention to regenerate alkylene oxide so that it can be recycled to the fluidized bed dehalogenation apparatus. It also is an object of this invention to maximize the production of alkylene oxide in the caustic scrubber and thereby minimize the production of alkylene glycol therein. A further object of this invention is to minimize or eliminate foaming in the caustic scrubber.